The present invention relates to a DC interrupting apparatus having a DC power source unit, a DC interrupter unit, and a load unit and, more particularly, a DC interrupting apparatus for preventing surge voltage from being propagated from a DC power source unit to a load unit and vice versa.
For a better understanding of the present invention, some problems involved in the conventional DC interrupting apparatus will be described with reference to FIGS. 1 and 2. FIG. 1 shows a conceptional view of a conventional interrupting apparatus for transmitting electric energy from a DC power source unit 1 to a load unit 3, through a DC interrupter unit 2. A DC current I fed from a DC power source unit 1 including an AC power source 4 and a rectifier 5 is supplied to the load unit 3, through an interrupter 6, for example, a vacuum interrupter. The load unit 3 is comprised of a resistor 7, an inductance coil 8 with a low resistance connected across the resistor 7, an inductance coil 9 magnetically coupled with the inductance coil 8, and a resistor 10 connected in series with the inductance coil 9 and consuming energy. When a DC current I is rapidly interrupted by the interrupter 6, a current flowing through the inductance coil 8 flows into the resistor 7. The current flowing through the resistor 7 attenuates in accordance with a time constant determined by an inductance of the inductance coil 8 and the resistor 7. The energy produced by a change of the current is propagated to the inductance coil 9 so that energy is supplied to the load 10. For the energy supply, a large current at a high voltage must be interrupted by the interrupter 6. To this end, usually, a plurality of interrupters are connected in series in the interrupter unit 2. Alternately, a plurality of groups each including a plurality of interrupters connected in series are connected in parallel. Because of the interruption of a large current, a steep surge voltage is frequently produced in the DC power source unit 1 and the load unit 3. Such a surge voltage is preferably absorbed in a unit at which the surge voltage is produced so as not to be transmitted to other units.
FIG. 2 shows a circuit diagram of a interrupter unit inserted between points A and B shown in FIG. 1. Inserted between the points A and B, a saturable reactor 11, a first interrupter 12a, a second interrupter 12b in series fashion. A series circuit including a capacitor 13a and a resistor 14a is connected across the interrupter 12a. Another series circuit having a capacitor 13b and a resistor 14b is connected across the interrupter 12b. Those series circuits are provided to absorb transient voltages applied across the interrupters 12a and 12b. Between the points A and B, a series circuit including a capacitor 15 and a normally opened switch 16 is connected and the capacitor 15 is previously charged by a charging device 17 to have polarities as shown.
In the interrupter unit shown in FIG. 2, when the DC current I is interrupted, the interrupters 12a and 12b are simultaneously opened to produce arcs across the respective interrupters. After a predetermined time lapse since these interrupters are begun to open, the switch 16 is closed and the capacitor 15 is discharged through the interrupters 12a and 12b and the saturable reactor 11. The discharging current is denoted as Ir (commutation current). When Ir&gt;I, a zero point occurs in a current flowing through the interrupters 12a and 12b. At the time that the zero point occurs, the current I is interrupted. In order that the interrupters 12a and 12b may easily interrupt the current I, it is desirable that the inclination (a changing rate of current) of a current flowing through the interrupters immediately before the current flowing through the interrupters becomes zero is small, and that a rate of increase of the voltage (recovery voltage) applied between the electrodes of each interrupters after the current is interrupted, is small. The saturable reactor 11 serves to make the inclination of the current small. That is, after the current is interrupted, the current flows through the saturable reactor is extremely small so that the reactor serves as a large inductance.
In FIG. 2, after a current flowing through the interrupters 12a and 12b is interrupted, an oscillating voltage is applied across terminals of respective interrupters for a relatively long time. As a result, after the DC current is interrupted and the time of several hundreds milliseconds is lapsed, the interrupter 12a, for example, is refired, while the interrupter 12b is not refired. At this time, the entire of the oscillating voltage is applied as an excessive voltage across the interrupter 12b. As a result, there possibly occurs a situation where the interruption is impossible in worst case.
During the period from the instance that an excessive voltage is applied across an interrupter until the interrupter is refired, much time is required. The time taken for the interrupter to be refired depends on the extinguish medium or the amplitude of the excessive voltage. This time may be several tens milliseconds or more in a vacuum interrupter. Within this several tens milliseconds, the insulation of the interrupter 12a first refired is recovered. With respect to the excessive voltage applied across the interrupter 12b, however, since charges stored in the capacitor 13b are not rapidly discharged, the excessive voltage applied to the interrupter 12b when the interrupter 12a is refired is continuously applied across the interrupter 12b. On the other hand, no voltage is applied across the interrupter 12a even if the insulation is recovered. Accordingly, when the interrupter 12a is insulation-recovered following the refiring, it is desirable that a part of an excessive voltage having been applied across the interrupter 12b is shifted to the terminals across the interrupter 12a and that the voltage applied across the interrupters 12a and 12b are equalized as rapidly as possible. A case where the interrupters 12a and 12b are connected in series has been described. When the number of the series-connected interrupters increases, the problems relating to the refiring, the insulation recovery and an excessive voltage being applied across an interrupter are more complicated.
The interrupter unit shown in FIG. 2 has additional following problems. A steep surge voltage occurring in the power source unit is propagated to the load unit, through the capacitors 13a and 13b. A steep surge voltage occurring in the load unit is likewise propagated to the DC power source unit. As a result, the performance of a DC interrupting apparatus may possibly be deteriorated.
In order to equalize the recovery voltage being applied across each of the interrupters 12a and 12b as rapidly as possible, it is conceivable that the resistors 18a and 18b are connected across the interrupters 12a and 12b, respectively, as shown in FIG. 3. In FIG. 4A, there is shown a relation between a voltage value (a relative value) applied across each interrupter in FIG. 2 and the lapse of time. Further, FIG. 4B shows a relationship between a voltage value (a relative value) applied across each interrupter shown in FIG. 3 and the lapse of time. In the figures, an original "0" of time represents a time point immediately after the DC current is shut off. At the time point "0", the interrupters 12a and 12b are both supplied with recovery voltages. At the time T1, for example, if the interrupter 12a is refired and the interrupter 12b is not refired, a recovery voltage is applied fully across the interrupter 12b as shown by V12b and no part of the recovery voltage is applied across the interrupter 12a as shown by V12a (FIGS. 4A and 4B). In FIG. 2, i.e. FIG. 4A, the capacitor 13b holds charges for a relatively long time. Accordingly, even if the insulation of the interrupter 12a is recovered, the voltage V12b across the interrupter 12b is held as it is, as shown in the figure. In FIG. 3, i.e. FIG. 4B, the interrupter 12a is refired at time T1 and, at time T2, is insulation-recovered, the charge in the capacitor 13b of the interrupter 12b is discharged so that the voltage across the interrupter 12b becomes V1 at the time T3. With this, the voltage across the interrupter 12a becomes V1 at time T3. In this case, it is desirable that the time constant of the circuit including the capacitor 13a and resistors 14a and 18a and that of the circuit including the capacitor 13b and the resistors 14b and 18b are small. When the capacities of the capacitors 13a and 13b are small, however, it is difficult to decrease the rising rate of a recovery voltage which is applied to respective interrupter after DC current interruption. Also, it is difficult to make the resistance of each resistor small in the light of the heat capacity of each resistor. Further, the FIG. 3 circuit can not prevent a surge voltage produced at the power source unit from being propagated to the load unit and the surge voltage produced at the load unit from being propagated to the power source unit.
Accordingly, an object of the present invention is to provide a DC interrupting apparatus which can prevent a surge voltage produced at the DC power source unit from being propagated to the load unit and a surge voltage produced at the load unit from being propagated to the power source unit.
Another object of the invention is to provide a DC interrupting apparatus which may shorten the time necessary to distribute a recovery voltage over series connected interrupters in an equal value.